The proposed research is designed to address specific questions of molecular and electronic structure, and chemical reactivity to gain a fundamental understanding of the catalytic activity of the Ni-containing protein acetyl coenzyme A synthase (ACS) through the synthesis, characterization and elucidation of reactivity of model complexes. This methodology will permit characterization of the intrinsic properties of structure and function of the metal sites at the molecular level of detail decoupled from the protein architecture. Recent results from structural biology have established details of the active site structure making now the optimum time to address this problem as the targets are well defined. Individual reactions will be systematically interrogated using the protocols of mechanistic inorganic chemistry including product analyses, kinetic measurements, stereochemical and radical clock probe investigations. The long-term goal of this project is to develop a detailed mechanistic understanding of how the structural, electronic and chemical properties of nickel sites in proteins are optimized for their intended catalytic transformations. The proposed research impacts our understanding of the biological implications of the essential trace element nickel that include the virility of Helicobacter pylori which has been associated with peptic ulcer disease, gastric carcinoma, and gastric lymphoma, and carcinogenesis through production of oxidizing species that degrade DNA. Additionally, acetogenic and methanogenic bacteria, organisms that contain ACS, may be important to human digestive function and dysfunction as they occupy a large volume of the colon. More broadly, a deeper understanding of ACS catalysis will advance fundamental understanding of redox-based Ni toxicity.